1. Field of the Invention
The present invention relates to medical devices for monitoring vital signs, e.g., respiration rate.
2. Description of the Related Art
Respiration rate (RR) is a vital sign typically measured in the hospital using either an indirect electrode-based technique called ‘impedance pneumography’ (IP), a direct optical technique called ‘end-tidal CO2’ (et-CO2), or simply through manual counting of breaths by a medical professional. IP is typically used in lower-acuity areas of the hospital, and uses the same electrodes deployed in a conventional ‘Einthoven's triangle’ configuration for measuring heart rate (HR) from an electrocardiogram (ECG). One of the electrodes supplies a low-amperage (˜4 mA) current that is typically modulated at a high frequency (˜50-100 kHz). Current passes through a patient's chest cavity, which is characterized by a time-dependent capacitance that varies with each breath. A second electrode detects the current, which is modulated by the changing capacitance. Ultimately this yields an analog signal that is processed with a series of amplifiers and filters to detect the time-dependent capacitance change and, subsequently, the patient's RR.
In et-CO2, a device called a capnometer features a small plastic tube that typically inserts in the patient's mouth. With each breath the tube collects expelled CO2. A beam of infrared radiation emitted from an integrated light source passes through the CO2 and is absorbed in a time-dependent manner that varies with the breathing rate. A photodetector and series of processing electronics analyze the transmitted signal to determine RR. et-CO2 systems are typically used in high-acuity areas of the hospital, such as the intensive care unit (ICU), where patients often use ventilators to assist them in breathing.
In yet another technique, RR is measured from the envelope of a time-dependent optical waveform called a photoplethysmogram (PPG) that is measured from the index finger during a conventional measurement of the patient's oxygen saturation (SpO2). Breathing changes the oxygen content in the patient's blood and, subsequently, its optical absorption properties. Such changes cause a slight, low-frequency variation in the PPG that can be detected with a pulse oximeter's optical system, which typically operates at both red and infrared wavelengths.
Not surprisingly, RR is an important predictor of a decompensating patient. For example, a study in 1993 concluded that a respiratory rate greater than 27 breaths/minute was the most important predictor of cardiac arrests in hospital wards (Fieselmann et al., ‘Respiratory rate predicts cardiopulmonary arrest for internal medicine patients’, J Gen Intern Med 1993; 8: 354-360). Subbe et al. found that, in unstable patients, relative changes in respiratory rate were much greater than changes in heart rate or systolic blood pressure, and thus that the respiratory rate was likely to be a better means of discriminating between stable patients and patients at risk (Subbe et al., ‘Effect of introducing the Modified Early Warning score on clinical outcomes, cardiopulmonary arrests and intensive care utilization in acute medical admissions’, Anaesthesia 2003; 58: 797-802). Goldhill et al. reported that 21% of ward patients with a respiratory rate of 25-29 breaths/minute assessed by a critical care outreach service died in hospital (Goldhill et al., ‘A physiologically-based early warning score for ward patients: the association between score and outcome’, Anaesthesia 2005; 60: 547-553). Those with a higher respiratory rate had an even higher mortality rate. In another study, just over half of all patients suffering a serious adverse event on the general wards (e.g. a cardiac arrest or ICU admission) had a respiratory rate greater than 24 breaths/minute. These patients could have been identified as high risk up to 24 hours before the event with a specificity of over 95% (Cretikos et al., ‘The Objective Medical Emergency Team Activation Criteria: a case-control study’, Resuscitation 2007; 73: 62-72). Medical references such as these clearly indicate that an accurate, easy-to-use device for measuring respiratory rate is important for patient monitoring within the hospital.
Despite its importance and the large number of available monitoring techniques, RR is notoriously difficult to measure, particularly when a patient is moving. During periods of motion, non-invasive techniques based on IP and PPG signals are usually overwhelmed by artifacts and thus completely ineffective. This makes it difficult or impossible to measure RR from an ambulatory patient. Measurements based on et-CO2 are typically less susceptible to motion, but require a plastic tube inserted in the patient's mouth, which is typically impractical for ambulatory patients.